Rocker John Mayer has stepped in to defend troubled actor Shia LaBeouf over his bizarre antics.

The Transformers star's strange behaviour has been grabbing headlines in recent days after he walked the red carpet at the Berlin International Film Festival in Germany with a paper bag over his head.

He also stormed out of a press conference after answering only one question, leaving journalists baffled by his bizarre response about a fishing trawler, and was back in the news again after he launched a live art installation in Los Angeles.

However, John Mayer is adamant the former child star is simply struggling to deal with his own success, writing in a series of posts on Twitter.com, "Re: Shia. Being young and very talented/successful is like dropping in to the top of Mount Everest via helicopter... There are moments in life where you need to feel quantum shift, movement. And if where you already are is considered the top... the dumb people disintegrate, and the smart people investigate. That investigation is never comfortable. Or all that pretty."

During his stint in the Los Angeles store, LaBeouf has been meeting visitors while sitting motionless with a brown paper bag over his head. The bag has the words 'I am not famous anymore' scrawled across the front and he has been repeating the phrase in a series of more than 20 posts on his Twitter.com page.

LaBeouf's antics have attracted ridicule from some Hollywood stars, including actor Jerry O'Connell, who has set up his own parody art installation next to LaBeouf's pop-up shop.